The degree to which an animal of either sex is choosy correlates exactly with the degree to which it invests in parental care. A black grouse, investing no more than sperm, is prepared to mate with anything that even resembles a female: a stuffed bird or a model will do. A male albatross, who will put all his best efforts into raising one female's young, is elaborately suspicious and selective, striving for the best female on offer. The overwhelming fascination of men with female youth argues that pair bonds have lasted lifetimes. Chimpanzees find old females just as attractive as young ones. The fact that men do prefer twenty-year olds adds one piece of evidence to the theory that Pleistocene man, like a modern man, married for life.
--Matt Ridley